When two elements unite in different proportions, by weight, to form more than one compound, Dalton supposed that (in most cases at any rate) one of the compounds is formed by the union of a single atom of each element; the next compound is formed by the union of one atom of the element which is present in smaller quantity with two, three, or more, atoms of the other element, and the next compound is formed by the union of one atom of the first element with a larger number (always, necessarily, a whole number) of atoms of the other element than is contained in the second compound; and so on. From this assumption, and the Daltonian conception of the atom, it follows that the quantities by weight of one element which are found to unite with one and the same weight of another element must always be expressible as whole multiples of one number. For if two elements, A and B, form a compound, that compound is formed, by supposition, of one atom of A and one atom of B; if more of B is added, at least one atom of B must be added; however much of B is added the quantity must be a whole number of atoms; and as every atom of B is the same in all respects as every other atom of B, the weights of B added to a constant weight of A must be whole multiples of the atomic weight of B.
The facts which were available in Dalton's time confirmed this deduction from the atomic theory within the limits of experimental errors; and the facts which have been established since Dalton's time are completely in keeping with the deduction. Take, for instance, three compounds of the elements nitrogen and oxygen. That one of the three which contains least oxygen is composed of 63.64 per cent. of nitrogen, and 36.36 per cent. of oxygen; if the atomic weight of nitrogen is taken to be 4.66, which is the weight of nitrogen that combines with one part by weight of hydrogen, then the weight of oxygen combined with 4.66 of nitrogen is 2.66 (63.64:36.36 = 4.66:2.66). The weights of oxygen which combine with 4.66 parts by weight of nitrogen to form the second and third compounds, respectively, must be whole multiples of 2.66; these weights are 5.32 and 10.64. Now 5.32 = 2.66 x 2, and 10.64 = 2.66 x 4. Hence, the quantities by weight of oxygen which combine with one and the same weight of nitrogen are such that two of these quantities are whole multiples of the third quantity.
Dalton's application of the Greek atomic theory to the facts established by the analyses of compounds enabled him to attach to each element a number which he called the atomic weight of the element, and to summarise all the facts concerning the compositions of compounds in the statement, that the elements combine in the ratios of their atomic weights, or in the ratios of whole multiples of their atomic weights. All the investigations which have been made into the compositions of compounds, since Dalton's time, have confirmed the generalisation which followed from Dalton's application of the atomic theory.
Even if the theory of atoms were abandoned, the generalisation would remain, as an accurate and exact statement of facts which hold good in every chemical change, that a number can be attached to each element, and the weights of the elements which combine are in the ratios of these numbers, or whole multiples of these numbers.
Since chemists realised the meaning of Dalton's book, published in 1808, and entitled, A New System of Chemical Philosophy, elements have been regarded as distinct and definite substances, which have not been divided into parts different from themselves, and unite with each other in definite quantities by weight which can be accurately expressed as whole multiples of certain fixed quantities; and compounds have been regarded as distinct and definite substances which are formed by the union of, and can be separated into, quantities of various elements which are expressible by certain fixed numbers or whole multiples thereof. These descriptions of elements and compounds are expressions of actual facts. They enable chemists to state the compositions of all the compounds which are, or can be, formed by the union of any elements. For example, let A, B, C, and D represent four elements, and also certain definite weights of these elements, then the compositions of all the compounds which can be formed by the union of these elements are expressed by the scheme An Bm Cp Dq, where m n p and q are whole numbers.
These descriptions of elements and compounds also enable chemists to form a clear picture to themselves of any chemical change. They think of a chemical change as being; (1) a union of those weights of two, or more, elements which are expressed by the numbers attached to these elements, or by whole multiples of these numbers; or (2) a union of such weights of two, or more, compounds as can be expressed by certain numbers or by whole multiples of these numbers; or (3) a reaction between elements and compounds, or between compounds and compounds, resulting in the redistribution of the elements concerned, in such a way that the complete change of composition can be expressed by using the numbers, or whole multiples of the numbers, attached to the elements.
How different is this conception of a change wherein substances are formed, entirely unlike those things which react to form them, from the alchemical presentment of such a process! The alchemist spoke of stripping off the outer properties of the thing to be changed, and, by operating spiritually on the soul which was thus laid bare, inducing the essential virtue of the substance to exhibit its powers of transmutation. But he was unable to give definite meanings to the expressions which he used, he was unable to think clearly about the transformations which he tried to accomplish. The chemist discards the machinery of virtues, souls, and powers. It is true that he substitutes a machinery of minute particles; but this machinery is merely a means of thinking clearly and consistently about the changes which he studies. The alchemist thought, vaguely, of substance as something underlying, and independent of, properties; the chemist uses the expression, this or that substance, as a convenient way of presenting and reasoning about certain groups of properties. It seems to me that if we think of matter as something more than properties recognised by the senses, we are going back on the road which leads to the confusion of the alchemical times.
The alchemists expressed their conceptions in what seems to us a crude, inconsistent, and very undescriptive language. Chemists use a language which is certainly symbolical, but also intelligible, and on the whole fairly descriptive of the facts.
A name is given to each elementary substance, that is, each substance which has not been decomposed; the name generally expresses some characteristic property of the substance, or tells something about its origin or the place of its discovery. The names of compounds are formed by putting together the names of the elements which combine to produce them; and the relative quantities of these elements are indicated either by the use of Latin or Greek prefixes, or by variations in the terminal syllables of the names of the elements.